The present invention relates to a technique for protecting copyrighted content by use of a license, and more particularly to an apparatus and a system for managing temporary licenses for content between a server and terminals.
Services have been proposed for delivering content such as movies and music through communication means such as the Internet and satellite broadcasting. Data handled by such a system is digitized and therefore is easy to duplicate, making it important to protect the copyrighted content. One proposed method for such protection is to encrypt the content before distributing it. For example, the content is encrypted through the public key cryptosystem, and at the same time, a decryption key is created for use to decrypt the encrypted content. Since the encrypted content cannot be reproduced without the corresponding decryption key, the decryption key and its use conditions may be managed as a pair, or a license, to prevent unauthorized use of the content and thereby protect the right of the content author, etc.
Japanese Patent Laid-open No. 2002-288448 discloses a system for reproducing content at a plurality of terminals owned by users, wherein after a license managed by the server is transmitted to a terminal, if the terminal has finished using the license, the license is deleted from the terminal or it is returned to the server so that the license can be transmitted to another terminal.
In content delivery service using the above copyright protection technique, it is important to employ a method capable of securely managing the delivered content licenses at the terminals.
In the above prior art technique, content data is encrypted, and a decryption key for decrypting the data and the use conditions of the key (hereinafter collectively referred to as a license) are used to protect the copyright. In this system, the license use conditions transmitted from the server to terminals are the same as those managed or held by the server, making it impossible to change the use conditions for each terminal. For example, a license including a “consumptive type” use condition whose value is updated at a terminal, such as the maximum allowable number of content reproduction operations, cannot be transmitted to a plurality of terminals at the same time.
Further, once transmitted to a terminal, a license must be returned to the server before transmitting it to another terminal, making it necessary that the server and the terminal (an apparatus or a memory device storing the license) communicate with each other when the license is returned. This means that the server must be aware of at which terminal the user has stored the license, which is inconvenient.
Still further, since each license transmitted to a terminal is managed by only attaching a flag to the corresponding license managed by the server, it is impossible to support the case where the use conditions vary at different terminals.